There are different technical areas wherein measuring the human body movements is necessary; one of them, which has reached a great economic importance in the last years is computer animation. Within this area, record of body movements for their application to animated characters is called Motion Capture. Such animation technique consists of recording the movements of an actor and mapping them to a character. The advantage of using Motion Capture over traditional animation is that it enables to accelerate the production since it avoids the use great groups of animators who manually generate the movements frame by frame. Another advantage of this technique is that it allows giving greater expressiveness to the characters, due to the fact that the movements are directly obtained from an actor. The fact of recording the position of certain body parts, the speed of their movements, subtle accelerations and their length, gives realism to the performance of the animated characters that would not be possible otherwise.
It is easy to imagine that computer animation is not the only area where Motion Capture can be applied. Since we can determine the movements of the body parts, it is possible to use this technique as an interface mean in control areas such as tele-robotics and tele-surgery.
There are different techniques for detecting body movements; among these, the most inexpensive and the ones that offer greater precision are the ones that use mechanical means for carrying out the measurements. The systems based on these types of techniques are called Mechanical Motion Capture Systems. Generally, these types of systems seek to measure the existing flexion among the moving body parts. The main difficulty is that connecting sensors to flexible parts of the body is required. Generally, such sensors are stiff electronic devices, such as potentiometers, encoders, etc.; therefore, their use is usually uncomfortable and not very practical.
The main object of the present invention is to allow the measuring of the movements of human or animal flexible bodies by mechanical means in a simple, cheap and comfortable way for the user of the device.
There are different methods in the state of the art for recording the human body movements with different purposes: computer animation, control, robotics, among others. This measuring and recording of the human body movements is called Motion Capture, and has reached importance in the last years due to its application in computer animation. A. Menache. Understanding motion capture for computer animation and video games. San Diego: M. Kaufmann: Academic, c2000.
In order to carry out an animation using motion capture, you simply record the relative or absolute position of each moving body part of an actor and such data are mapped to the moving body parts of a character.
There are different motion capture systems, which, according to the techniques used can be classified in three great groups: optical, magnetic and mechanic.
Optical systems use markers over the moving parts of a body, such markers are recorded by cameras from different points of view so that afterwards, by means of triangulations, to infer the tridimensional position of each one of these markers within a certain space. One of the problems of this type of systems is that the space where the markers can be recorded is limited by the number, the position, visual field and resolution of the cameras. Such space should also have an adequate illumination and if possible, there should not be any objects obstructing the visibility of the cameras.
Despite maintaining a controlled environment within the capture space, sometimes it is not possible to avoid the occlusion of the markers by the actors body parts. The occlusion can be avoided by increasing the number of cameras, thus covering a bigger number of points of view, but therefore increasing the necessary computer power to calculate the position of the markers. The calibration of these systems can also takes longer since you have to know the position of each of the cameras and to calibrate each one of them with a series of pattern marks. Due to the complexity and the amount of equipment needed, these systems are usually very expensive.
The main advantage of optical systems is that the user does not need to carry any type of sensors, but only clothes of a color contrasting the color of the markers, so that he/she is able to freely move within the capture space. Another advantage is that it is possible to capture the motion of multiple actors at the same time and within the same capture space.
Magnetic systems use a series of electromagnetic field emitters that send signals at different frequencies, which can be detected within a certain field of action by a series of sensors. The principle used by these systems is that the field intensity decreases evenly and proportionally to the distance between emitter and receiver, therefore it is possible to calculate the existing distance between a field emitter and a sensor; the emission frequency of said field is useful to distinguish the specific field it is about.
Knowing in advance the position of each field emitter, it is possible to calculate by triangulation the position of each sensor, so that each sensor is fixed on each body part which position must be known. The main disadvantage of these types of systems is that it is very sensitive to the interference generated by metallic objects that alter the electromagnetic field between the emitter and the receiver, so that it is required to keep such area within a controlled environment. Another disadvantage is that the measurements, such as with optical systems, are also limited to a specific area. The advantage of these systems compared to optical systems is their price.
Finally, mechanical systems are those based on direct measuring, by means of mechanical contact, of the existing flexion between two moving body parts, such as the joints. This flexion generally represents a rotation or relative displacement between one part and the other. In order to be able to carry out the measurements, the sensors are directly arranged on the body forming a sort of exoskeleton on the same. Generally, the direct measuring of the body movements is more precise, so this is why this is the method used in tele-surgery, at a lower cost. However, the use of sensors on the body can lead to an obstruction problem if it is not adequately treated. The simplest systems arrange rigid electronic devices such as potentiometers, encoders, etc., on the moving parts of the body, all of these subject to a rigid exoskeleton that guarantees the correct position of such sensors. It is easy to observe that this methodology can be uncomfortable. However, there are more complicated and expensive systems that use flexible sensors like optic fibers and stress-measuring calipers, which are also put on the moving body parts by means of tight clothes, thus forming a flexible exoskeleton. Even though using flexible sensors produces a little accuracy lost when measuring the movements, it is also true that such accuracy is not necessary concerning the animation field.
The system of the present invention solves the existing problems in the motion capture systems, by retaking the advantages of the mechanical systems with flexible sensors and combining it with the advantages of the optical systems, but at a lesser cost.
The methodology used is based on detection and transmission of the movements of the body by mechanical means and measuring each one of them by optical means. With this mixed procedure, the disadvantages inherent to both applied methodologies are eliminated.